In providing freshly mixed concrete to construction sites, it is necessary that the various ingredients for making the concrete be mixed in accurate proportions and for the proper length of time to ensure that the resulting concrete will meet the specifications of the particular construction job. Concrete mixing trucks are often used for this purpose, but suffer from several disadvantages. Such trucks are only efficient when used in the simultaneous mixing and transportation of a large amount of concrete. The concrete must be ready-mixed, as the trucks have no metering capabilities. This requires an expensive stationary proportioning plant for metering all the concrete materials. When the contents of the mixing drum are greater than the concrete requirement at the job site, the remainder must be quickly unloaded, often wastefully. If a loaded truck breaks down, it is necessary to manually unload the concrete before it can set inside the drum. The distance from the stationary plant to the job site is limited by the maximum mixing time of the concrete. Additionally, the time spent by the trucks in travelling between the job site and the stationary plant is non-productive.
One solution to the disadvantages of the stationary plant and mixing truck combination described above is the on-site erection of a portable batch plant. Such plants are typically very large, cannot be transported as a single unit, and require extensive on-site erection work. The on-site mixing of dry materials, particularly cement can also release large quantities of dust into the air, creating an air-polution problem. U.S. Pat. No. 3,064,832 to Heltzel shows such a portable plant. While the plant of Heltzel is transportable as a single unit, it still requires on-site erection time of approximately 22/3 hours (column 1, line 19). In addition, the Heltzel plant has no mixing drum, and thus still requires the use of a mixing truck. Plants of a similar nature to Heltzel are provided by U.S. Pat. Nos. 3,998,436 to Allen, et al.; 3,295,698 to Ross, et al.; and 3,945,619 to Taibi. Portbble batch plants shown by U.S. Pat. Nos. 4,189,237 to Bake, 4,339,204 to Placzek, 4,285,598 to Horton, and 3,746,313 to Weeks each provide a single mixing drum as part of the plant. Having only a single drum effectively reduces the capacity of these plants, however, since a relatively large proportion of available equipment time must be spent waiting for one batch to finish the mixing step before continuing preparation of the next succeeding bath. This discontinuous plant operation is a disadvantage where job requirements demand a continuous supply of concrete.